A dormer window is classified as an alteration to to a dwelling and not an extension. Therefore the 40 sq.m. exemption does not apply. Furthermore, a dormer window would normally not fall under exepted development as defined under Section 4 (1)(h) of the Planning and Development Act (although in the case of your house having authorised dormers already and also your neighbours’ houses it may be that planning would not be required as the addtition of another dormer may not render your dwelling inconsistent with the appearance of the dwelling as existing or with neighbouring dwellings). So planning permission would more than likely be required. A Section 5 declaration is always a good thing to get if your confused. €80 cost and takes 4 weeks only.